The disclosure relates to fiber optic connectors, and more particularly to connector tuning methods for fiber optic connectors and ferrules permitting tuning of fiber optic cable assemblies.
Optical fibers are useful in a wide variety of applications, including the telecommunications industry for voice, video, and data transmission. In a telecommunications system that uses optical fibers, there are typically many locations where fiber optic cables that carry the optical fibers connect to equipment or other fiber optic cables.
To conveniently provide these connections, fiber optic connectors (“connectors”) are often provided on the ends of fiber optic cables. Fiber optic connectors are used to optically connect one optical fiber to another, or to connect an optical fiber to another device such as an optical transmitter or an optical receiver. A fiber optic cable typically carries the optical fiber. The connector and the fiber optic cable constitute a cable assembly. The connector is typically formed by engaging an inner housing with an outer housing, wherein the inner housing supports a ferrule.
An important property of a connector is its ability to provide an efficient optical connection, i.e., an optical connection whereby the optical loss (also called “insertion loss”) due to the connection is minimal. This efficiency is referred to in the art as the “coupling efficiency.”
It is advantageous to “tune” connectors in a factory where connectors are assembled to minimize optical loss in the field. The tuning process involves detection of the fiber-ferrule concentricity (also referred to as “fiber core to ferrule” concentricity), i.e., the offset between the optical fiber core and the true center of the ferrule in which the optical fiber is supported. A known method for tuning involves measuring insertion loss of a connector mated to a master connector having a fiber core position of known magnitude and direction with the master core direction being relative to a mechanical key. The connector being tested may be rotationally “tuned” to maximize optical throughput and locked in the as-rotated position.
Tuning may also be performed using other contact methods that do not involve making a connection to a master connector. Alternatively, non-contact methods may be employed to determine fiber-ferrule concentricity (e.g., such as disclosed in U.S. Patent Application Publication No. 2015/0177097, which is hereby incorporated by reference herein). The contact methods not involving connection to a master connector, as well as the non-contact methods, typically require a substantial portion of the outer surface of the ferrule to be exposed. Because the inner housing in most connector designs covers substantially all (e.g., about 90%) of the ferrule length, these measurement methods normally require the fiber-ferrule concentricity to be measured without the inner housing in place. Accommodating such a requirement in cable assembly processes may add costs, complexities, and/or inefficiencies.
Some early connector designs included external keys that were installed to set the “tuned” position. Later connector designs allowed a housing to be installed after tuning, and still later connector designs provided for a fiber-ferrule subassembly to be pushed rearward (i.e., axially displaced) to enable rotational indexing of the ferrule.
Connector designs that use separate external keys have been mostly abandoned due to their size, cost, and complexity. Installing an inner housing after tuning is also not preferred, since excess fiber generally needs to be absorbed by the cable structure, and it can be inefficient to measure fiber-ferrule concentricity without an inner housing in place. Tuning by indexing a ferrule when axially displaced also has the drawback in that an end user may inadvertently alter the tuning position set at the factory.